The present invention relates generally to communication systems and, more particularly, to a slotted CSMA communication system arrangement wherein periodic and aperiodic messages are prioritized prior to transmission by a plurality of remote terminal units while eliminating disruptive collisions therebetween.
Communications which process data information are enjoying ever increasing usage. Data only systems are becoming more prevalent but others may accommodate both data and voice. Typically such data systems comprise a base station with full duplex capability, a network control processor (NCP), a system host computer, and a plurality of portable radio data terminals operating essentially in half duplex mode.
In these data systems, suitable channel access protocol techniques are required to minimize interference as between the remote radio data terminals themselves and to keep the system operating at optimum efficiency. Many methods, or protocols, have been developed to address this problem, all with certain advantages and certain disadvantages.
For example, one such protocol that has enjoyed wide-spread application, is known in the art and is commonly referred to in the literature as "non-persistent busy tone (bit) multiple access". In essence, this protocol arrangement permits channel contention between radio data terminals and, when one such terminal gains channel access and begins to transmit data, the central station or controller informs/advises the other terminals in the system of such circumstance by way of setting "inhibit bits" at predetrermined locations or positions in the outbound data stream. When a particular data terminal encounters these inhibit bits, it waits for some period of time, for example, a random time, before contending again for access to the channel. In this way, the system operates with minimum or at least reduced interference and an orderly process is established for all participating data terminals in the system to utilize the available system capacity.
This system control technique, also referred to as slotted CSMA contention protocol, has a number of variations, such as non-persistent slotted CSMA, persistent slotted CSMA, p-persistent slotted CSMA, and virtual time slotted CSMA. In any event, however, this CSMA contention protocol arrangement, and its many variations, effectively schedule transmissions from the remote device population to minimize collisions between messages on the inbound channel and also minimize the unused space between messages. However, such collisions do occur and diminish the efficiency and reliability of the system. Moreover, all such messages are treated equally and the remote unit wishing to transmit, whether it be a reocurring or periodic, message, or simply an aperiodic message, must nevertheless contend for access to the inbound channel with all of the other remote units.
It will be appreciated then, that what is needed is a protocol or traffic control arrangement which includes mechanisms for prioritizing access to the inbound channel so as to prevent these destructive and unwanted collisions between inbound messages, whether they be periodic or aperiodic and to arrange the traffic in such a way that the aperiodic message may be utilized to fill in the gaps between the other or regularly occurring messages being transmitted by the system data terminals on the inbound channel.